6.5 Medical Physics (Ultrasound)

Question 1

What is ultrasound?

Answer 1

A longitudinal sound wave with a frequency greater than human hearing range (+20kHz) (medical typically uses MHz)

Question 2

Benefits of ultrasound

Answer 2

+ Non invasive
+ Non ionising
+ Quick and Affordable

Question 3

What does a Piezoelectric Material do?

Answer 3

Generates voltage when it is contracted or expanded, or will contract/expand if a voltage is applied.

Question 4

I have applied voltage to a piezoelectric crystal. What will it do?

Answer 4

It produces ultrasound vibrations.

Question 5

A piezoelectric crystal has absorbed ultrasound vibrations. What does it do?

Answer 5

It produces an alternating voltage.

Question 6

How do ultrasound transducers work? [3]

Answer 6

• They have an alternating potential difference which causes repetitive compression and stretching of the crystal.
• This is done at a resonant frequency, to increase intensity.
• Once ultrasound has been created, the potential difference is turned off and the reflected signal is read.

Question 7

What are the two types of ultrasound scan?

Answer 7

A Scan
B Scan

Question 8

How does an A scan work?

Answer 8

A single transducer is used to emit a signal, then later receive the reflected signal back.

Question 9

What is an A scan used for, and how?

Answer 9

Determining the distance from the ultrasound device to the point of reflection. This can be done by measuring the time delay between generating and receiving the signal, and using the speed of sound in the media to approximate the distance.

Question 10

Where are ultrasound waves usually reflected?

Answer 10

The boundary between two media.

Question 11

How does a B scan work?

Answer 11

A transducer is moved over the patient’s skin. At each position, the scan produces a measure of the time interval, and so the distance to the reflection point between signal production and reception.

Question 12

What is a B scan, in comparison to an A scan?

Answer 12

The B scan is a series of A scans that are stitched together to form an image.

Question 13

In ultrasound scanning, why are ultrasound waves pulsed?

Answer 13

To allow time for the reflected waves to be received.

Question 14

What is the advantage of using smaller wavelengths in ultrasound scanning?

Answer 14

They give more detailed images, as it allows the soundwaves to diffract around finer points of detail.

Question 15

What is the acoustic impedance, Z, of a sound wave?

Answer 15

The product of its density, and the speed of sound in that medium.

Z = pc [kgm⁻²s⁻¹]

Question 16

What happens when ultrasound hits a boundary between two media?

Answer 16

A fraction of the wave energy (or intensity) is reflected. The rest passes through and is transmitted.

Question 17

Some ultrasound hits a boundary between two media. What does the fraction of waves reflected depend on?

Answer 17

The acoustic impedances of the media.

Question 18

State the equation for determining the reflection coefficient.

Answer 18

Iᵣ / I₀ = (Z₂ - Z₁)² / (Z₂ + Z₁)²

Iᵣ : Reflected intensity
I₀ : Original Intensity
Z₁: Impedance of first medium
Z₂: Impedance of second medium
[Iᵣ / I₀ : Reflection Coefficient]

Question 19

What is the reflection coefficient?

Answer 19

The fraction of the intensity reflected at the boundary.

Question 20

When does the equation for determining reflection coefficient apply only?

Answer 20

Only when the sound waves are travelling perpendicular to the boundary, so no refraction occurs.

Question 21

Looking at the equation for the reflection coefficient, what happens when Z₁ is similar to Z₂?

Answer 21

The fraction of intensity reflected is small, so most of the wave energy is transmitted.

Question 22

Looking at the equation for the reflection coefficient, what happens when Z₁ is very different from Z₂?

Answer 22

The fraction of intensity transmitted is small, so most of the wave energy is reflected.

Question 23

What immediate problem is there with operating an ultrasound transducer in air next to the patients skin?

Answer 23

There will be a large amount of reflection due to the differences in the impedances of the two media.

Question 24

How do we maximise transmission of ultrasound into a patient (from the air past their skin)?

Answer 24

Apply a gel which has a similar impedance to the skin, onto the patient’s skin.

Question 25

What is the doppler effect?

Answer 25

The change in frequency of a wave when it is reflected or produced by a moving source.

Question 26

What is doppler imaging?

Answer 26

A non-invasive technique that measures the speed of blood flow.

Question 27

How does doppler imaging work?

Answer 27

• Ultrasound waves are sent into a blood vessel.
• The blood flowing past the transducer contains iron that reflects the wave back to the transducer.
• Depending on the direction and speed of flow, the ultrasound frequency is either shifted up or down.

Question 28

State the doppler imaging equation

Answer 28

Δf = 2fvcosθ / c

Δf: Observed frequency shift
f : Original frequency
v : Speed of blood flow
c : Speed of ultrasound in blood
θ : Angle between probe and direction of blood flow.

Question 29

What is doppler imaging extremely useful for? [3]

Answer 29

• Showing blood flow through arteries and veins
• Revealing blood clots and narrowing artery walls
• Calculating the volume of blood flow.